1. Background of the Invention
Flip chip technology is the name of a process in which a semiconductor chip is flipped over so that the active side with the connection pads faces towards the substrate. In a high volume manufacturing process. Alignment marks on the chip and the fiducial marks on the substrates are used for the fast automatic alignment between the chip and the substrate using a flip chip tool.
Flip chip technology is the fastest growing chip interconnect technology as it allows the largest numbers of input/outputs (I/Os) for the smallest footprint of the chip. This allows small packages including packages such as chip-scale packages.
Gruber et al. in their paper “Low-cost wafer bumping,” IBM Journal of Research and Development, IBM JRD 49-4/5 (http://www.research.ibm.com/jpournal/rd/494/gruber.html) (Aug. 16, 2005) describe flip-chip solder-bump interconnections as the face-down soldering of integrated circuit (IC) devices to chip carriers by means of conductive bumps on the chip bond pad. The use of this bump technology also extends to passive filters, detector arrays and MEMS devices. IBM introduced this technology in the early 1960's with the solid logic technology in the IBM System/360™. It extended interconnection capabilities beyond existing wire-bonding techniques, allowing the area array solder-bump configuration to extend over the entire surface of the chip (die) providing solder bumps for interconnection to a substrate by the C4 (controlled collapse chip connection) solder reflow process developed by IBM. This allowed for the highest possible I/O counts to meet the increasing demand for electrical functionality and reliability in IC technology.
The original wafer-bumping process of metal mask evaporation in which ball-limiting metallurgy (BLM) also known as under board metallization, or under mask metallization, or under mask-bump metallurgy (UBM) involve the evaporation onto a wafer surface of solder through mask openings in an area array fashion. The need for increased I/O density and count, and pressures to lower the cost of flip-chip interconnections have spurred the development of other wafer bumping techniques such as electroplating or stencil-printing/paste-screening (solder paste) bump processes. Some of the more newly developed bumping processes include transfer printing, solder jetting, and bumpless and conductive particle applications.
In its broader aspect, BLM, or UBM comprises the application of a metal coating to the die contact pads such as aluminum or copper contact pads, where the metal coating provides a surface that can adhere to solder. One process involves cleaning an aluminum terminal pad followed by activation to remove any aluminum oxide layer on the pad and applying a thin layer of zinc by means of a single or double zincate coating. By following this coating with an electroless Ni (P) plating process the zinc is replaced with nickel which forms a strong bond between the nickel and aluminum. Manufacturers use a similar process for a copper terminal pad with the exception that they clean the pad first with a dilute etchant, followed by activation with a palladium dispersion or solution and then apply the electroless Ni (P) film, or alternatively an electrolytic or sputtered Ni (V) film. Subsequent steps involve application of solder, e.g., solder “bumps” to the metal coating. Gruber et al. (supra) give a detailed explanation of all of the foregoing processes.
The so-called “solder bumps” provide a space between the chip and the substrate after flip chip assembly, usually filled in the last steps of the assembly process with a nonconductive “underfill” material that adhesively joins the entire surfaces of the chip and the substrate. The underfill not only blocks other contaminants from entering into the structure but also locks the chip and substrate to one another so that differences in thermal expansion do not break or damage the electrical connections of the bumps.
New solder-bumping technologies have developed that include some of the attributes of plating (extendibility to larger wafers and smaller bump size/pitch) and solder paste screening (flexibility of Pb-free alloy selection and low cost). One process comprises injection-molded solder (IMS) technology developed at IBM Research as an outgrowth of earlier work using solder for high performance thermal joints.
Industry nonetheless pressed for lower cost, higher quality bumping processes, and to this end IBM developed the C4NP (C4 new process, announced on Sep. 13, 2004) for IMS wafer bumping processes. Gruber et al. (supra) describe this process which basically involves processing a wafer and a solder mold simultaneously, but in separate processes that subsequently merge. Solder fills the mold in a prearranged pattern, after which the filled mold goes through an inspection step and alignment step with a wafer. Prior to alignment the wafer undergoes BLM deposition and patterning followed by inspection. After aligning mold and wafer the assembled components go through a reflow process to transfer solder to the wafer.
The overview of flip chip technology shows its major advantage lies in utilizing the total chip area to make the I/O connections, whereas wire bonding uses only the chip periphery. A disadvantage of flip chip technology is that stresses that arise from the thermal mismatch between the silicon (chip) thermal expansion coefficient (CTE) and the CTE of the substrate are borne fully by the solder bumps (C4s) used to make the interconnect between chip and substrate. As noted, in order to ameliorate the stresses flip chip packages are usually underlined, i.e., a resin is placed between the chip and the substrate and acts as encapsulant of the solder bumps and an adhesive between chip and substrate. The effect of such underfills is that the long-time reliability of underlined flip chip packages is greatly enhanced compared to counterparts without an underfill.
IBM invented the OBAR (Over Bump Applied Resin) method in which pre-applied underfill on the chip for flip chip assembly eliminates the stress induced failure of the back-end-of-line (BEOL) during the flip chip assembly process. However, when the underfill is pre-applied on the chip, the alignment between the chip bumps and the substrates pads (or pre-solder bumps on the substrate) is very difficult because the OBAR covers the alignment marks and bumps on the chip and they cannot be used for the alignment. Therefore, very good transparency of the OBAR material is needed for the successful flip chip assembly process.
In joining chip to laminate pads either with pre-applied underfill on the presoldered bumps on laminate pads or OBAR pre-applied underfill on the wafer (chip), the process window to achieve a successful joining structure is complicated because two totally different material systems, the solder and resin, need to work together and be compatible with each other. Optimization of the process parameters, such as temperature profile, ramp rate, forces, resin formulation that affect curing, and flow of pre-applied underfill, and the like, are all critical to determining the quality of the final joint. There are several wafer-level pre-applied underfill processes, among them the Wafer-level Underfill (WLUF) process which uses an over-bump wafer-applied resin, that is then B-staged, followed by dicing of the wafer to singulate chips and finally joining the chips with the WLUF layer to substrates. The WLUF process has been described by Buchwalter, Feger, Hougham, LaBianca, and Shobha, U.S. Pat. No. 6,919,420.
The WLUF process, however, has a few drawbacks such as the fact that the WLUF material must be applied to the full wafer before chip singulation. This requires wafer testing to identify bad chip sites and wafer-level burn in, if chip burn in is required before the chip is joined to the package. While these processes are possible, they require special tooling and thus add cost to the package.
Further, functioning and non-functioning (a.k.a. good and bad) chip sites are coated during the wafer-level underfill apply process. The materials and processing cost for the bad chip sites through singulation has to be added to the cost of making the good WLUF coated chips. This cost can be significant, if the yield of the wafer is low, i.e., at the beginning of a new program or for complex chips.
Additionally, application of the WLUF material to the wafer requires storage until the wafer can be diced and further storage until the WLUF coated chips can be joined to the package. This requires stability of wafer-applied WLUF material for up to 6 months and may require storage of such wafers under nitrogen or other special conditions.
The WLUF layer may also obscure the C4 pattern and/or other alignment marks making it difficult to align WLUF coated, diced chips to the substrate before joining. The WLUF in this regard may contain high pigment or filler loading in order to ameliorate the difference in the coefficient of thermal expansion (CTE) of the semiconductor chip and the circuit board joined to it but this has other consequences. Increased pigment or filler loading further opacifies the WLUF making it difficult to ascertain alignment features or markings on the board and/or chip.
While these obstacles can be overcome as described by Buchwalter et al. (infra) and, U.S. Pat. No. 6,919,420 (supra), it would be of great advantage, if a process could be developed that could be applied to a singulated chip prior to joining. This would be of particular advantage for multi-chip modules and CSP (chip-size package) applications.
Investigating the use of the OBAR WLUF process as a pre-applied underfill on a substrate over tall substrate bumps will aid in defining an optimized process window to achieve good joining. The present invention, however, broadens the process window so good solder joints and fillets, i.e., underfill that extends beyond the periphery of the substrate or chip, can be achieved more easily. The advantage of an OBAR pre-applied underfill on a substrate over the OBAR WLUF process is that the former does not affect chip burn-in and testing processes.
P. Gruber, P. Lauro, J. W. Nah, K. Toriyama, U.S. Pat. Pub. No. 2010/0116871 entitled “Injection Molded Solder Method for Forming Solder Bumps on Substrates” shows how the tall substrate bumps can be formed on a substrate.
Several methods are disclosed to see the alignment marks after applying OBAR on the wafer (US 2009/0102070 A1 and US 2008/0265445 A1) but these methods need additional process steps to remove the B-stage cured OBAR.
In the case of pre-applied underfill on a substrate, it can be dispensed only on the pre-solder bumps area on the substrate so there is no issue of automatic alignment by using the alignment marks on the chip and the fiducial marks on the substrate.
However, the underfill must be dispensed one substrate by one substrate just before assembling the flip chip which decreases the throughput. Also, the method of pre-applied underfill on a substrate may increase the filler inclusions because a thick pre-applied underfill should be dispensed due to the small height of the pre solder bumps on the substrate which is less than half of the height of the solder bumps on the chip.
Accordingly, to get the advantage of the pre applied underfill which eliminates the stress induced failure of the BEOL, it is generally desirable to have a new method for flip chip manufacturing with pre applied underfill which enables the fast automatic alignment for high throughput and a short distance of underfill through which the solder must be pushed so that filler inclusions inside the interconnections can be minimized or eliminated.
2. Related Art
The following references comprise related art teachings:    C. Feger, N. LaBianca, M. Gaynes, S. Steen, “The Over-Bump Applied Resin Wafer-Level Underfill Process: Process, Material and Reliability,” IBM Research Report, RC24851 (W0908-142) Aug. 31, 2009    S. Buchwalter, C. Feger, G. Hougham, N. LaBianca, and H. Shobha, U.S. Pat. No. 6,919,420.    S. L. Buchwalter, D. Danovitch, F. E. Doany, P. A. Gruber, R. Iyengar, N. C. LaBianca, U.S. Pat. No. 6,924,171.    C. Feger, N. C. LaBianca, G. Hougham, H. K. Shobha, and S. L. Buchwalter, “A Wafer-level Underfill Process for Flip-chip Packaging,” Proc. IMAPS Flip Chip Tech. 2003 (Feger et al.).    R. Mahidhara, “Comparing Chip-Scale Packaging to Direct Chip Attach,” Chip Scale, May-June, 1999    L. Crane, D. Gamota, R. W. Johnson, and P. Neathway, “Making Direct Chip Attach Transparent to Surface Mount Technology,” Chip Scale, September-October, 1999 B. Ma, E. Zhang, S. H. Hong, Q. Tong and A. Savoca, “Material Challenges for Wafer Level Packaging”, Proc. Int. Symp. on Adv. Packag. Materials Processes, Properties and Interfaces P. 68, 2000.